Explanation

(a)
$Fe(OH)_{3}$is a positively charged sol, thus coagulated by negative ion (anion). Smaller the charge on anion, smaller is its coagulating power or higher is its flocculation value.
$(a)KBr \rightarrow K^{+} + {Br}^{-}$
$(b)K_{2}{SO}_{4} \rightarrow 2K^{+} + {SO}_{4}^{2 -}$
$(c)K_{2}{CrO}_{4} \rightarrow 2K^{+} + {CrO}_{4}^{2 -}$
$(d)K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack \rightarrow 4K^{+} + \left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$
$\because{Br}^{-}$has smaller charge.
$\therefore KBr\$is least effective in coagulating $Fe(OH)_{3}$ sol.